Precise pressure measurements are necessary in a variety of technological fields. Among the common areas of application for pressure measurement technology are oil and gas processing, such as instrumentation for measuring pressure in oil wells and oil pipelines, monitoring of pressure in industrial and consumer liquid processing devices, such as boilers and the like, medical equipment manufacturing, and various weight measurement instruments, such as scales and the like, that rely on pressure for determining weight, and many other fields of use.
For many applications, measuring pressure requires high precision that is not readily available within the present state of the art, or is expensive thus limiting the market reach of the technology at issue. For example, laboratory scales that provide high precision rely on complex components and therefore are expensive.
The instrumentation for determining pressure of oil wells is one of the principal applications for the pressure measurement technology. For oil-producing industry, the pressure inside an oil well is an important parameter of interest, the monitoring of which allows improvement of the yield from the well. It is especially important in maximizing the lifetime production of the well. At the same time, the pressure sensing components of the pressure measurement system must be placed deep inside the well, where environmental conditions, such as temperature and pressure, are very challenging. Also, the replacement and service of the pressure sensing components is a complicated and expensive process. Therefore, in addition to usual requirements of cost and ability to withstand conditions inside the well, the pressure sensors designed for use in oil wells preferably should be reliable, have a long useful lifetime, and minimal service requirements.
The need for improved pressure measurement technology is widely recognized. The precise measurement of pressure is a challenging goal in and of itself. A variety of pressure sensors are available in the marketplace. One type of conventional sensors is a spring-loaded sensor having a spring that provides a biasing force against the pressure being measured. Such sensors operate by balancing a load again a known biasing force of the spring, and determining the amount of spring deflection and the corresponding pressure exerted upon the spring. The spring-loaded sensors have a number of known disadvantages, such as a relatively low degree of accuracy and the need for repeated re-calibration. Also, it is believed that the spring-loaded sensors cannot reliability operate at high pressures and temperatures.
Other known sensors are piezoelectric transducers, which are often used in pressure gauges and scales. The piezoelectric pressure sensors operate by measuring the electric signal produced in response to changes created by the pressure being measured in a crystal lattice. The piezoelectric transducers also have known disadvantages. They suffer from a need for re-calibration and a relatively short operating life. They also may be expensive and may not have sufficiently high precision for certain applications. The piezoelectric transducers also are believed to require temperature compensation even at modest temperatures due to substantial differences in temperature expansion coefficients of various elements of the transducer and sensor assembly.
For these and other reasons, various forms of optical sensor technology are more and more prevalent in challenging pressure measurement applications. Wave-guide elements, such as optical fiber, are often used as sensing or transmitting structures, or both, in optical sensors.
Generally, fiber optic sensors may be classified as either “intrinsic” or “extrinsic.” The “intrinsic” fiber optic sensors rely on properties of the optical fiber itself to measure pressure or other environmental parameters. In addition to pressure, optical fiber sensors include fluid level sensors, temperature sensors, fiber optic gyroscopes, and the like. One type of “intrinsic” optical fiber sensors utilizes a portion of optical fiber having in-core fiber grating, such as Bragg grating (FBG), which functions as an element upon pressure is exerted. The Bragg grating may be formed by doping the optical fiber with various suitable materials (e.g., Ge), and then exposing the doped fiber to an interference pattern, thus producing variations in the refractive index of the fiber transmission core.
An example of the pressure sensor that uses Bragg grating fiber is disclosed in U.S. Pat. No. 6,034,686. The '686 patent discloses several embodiments of intrinsic optical sensors for measuring differential pressure. Essentially, in all of the embodiments of the '686 patent, an FBG element is set transversely to the direction of the pressure. The FBG portion of the fiber optic cable is interrogated with a light source and a detector, with the sensing FBG portion of the fiber being under the transverse strain. The spectral characteristics of the light that passes through the FBG sensing portion vary as a function of the transverse strain, and therefore the pressure, which may then be determined from the spectral analysis. Typically, various spectral demodulation systems such as Fabry-Perot filters, optical spectral analyzers and the like are coupled to the fiber as detectors to interpret the magnitude of spectral changes, with the signal being processed in a usual manner to calculate the strain and the differential pressure. Such spectral sensing methodology may be excessively complicated and require expensive detecting devices.
Other intrinsic fiber optic sensors are disclosed in U.S. Pat. No. 5,714,680. The '680 patent describes an embedded fiber optic sensor having a fiber Fabry-Perot interferometer embedded into a metal part that is located in a housing. Pressure, acting on one end of the metal part, compresses the metal part, with a magnitude of compression sensed by the Fabry-Perot interferometer, thereby providing a measure of the pressure.
Other types of fiber optic pressure sensors are “extrinsic” sensors. In these sensors, the fiber optic cable is used either as a transmitting structure to couple the portion of the device in direct contact with pressure, to a processing station, or to translate the pressure exerted upon a some type of mechanical element into spectral information, and ultimately into an electric signal.
A common type of such sensing element is Fabry-Perot interferometer. Typically, the Fabry-Perot interferometers include two parallel reflective structures facing each other with one of the reflective structures being capable of deflection or movement. When the pressure is applied to the reflective structure capable of deflection and light is passed between the structures, there occurs a change in the interference pattern created by the reflective structure. The change in the interference pattern is a function of the magnitude of deflection. U.S. Pat. No. 5,128,537 describes a pressure sensor that uses a Fabry-Perot interferometer. The '537 patent describes a pressure sensing system that includes two parallel mirrors, one of which is attached to a flexible diaphragm and another is attached to a transparent plate. A light is passed into the space between the mirrors, creating an interference pattern through the transparent plate. The change in the position of the mirror attached to the flexible diaphragm is interpreted on the basis of the changes in the interference pattern.
Many of the presently available optical sensors have a number of disadvantages, some of which are inherent in their construction. Thus, many devices must utilize expensive parts, such as optical spectral analyzers, because of the nature of the pressure-sensing methodology.
Another common disadvantage is the necessity for temperature compensation. In general, the changes in pressure are converted into mechanical movement, which in turn causes changes in either the length or the cross-section of the sensing optical fiber portion. To provide this mechanical movement information to the fiber or other similar structure, most optical sensors rely on direct contact between the fiber and the mechanical part that moves or deflects as a function of pressure (e.g., a diaphragm of the '537 patent or FBG-contacting surface(s) of '686 patent).
However, for many applications, especially in high pressure and temperature applications such as oil well sensors, the position of the flexible mechanical element, and therefore the signal received by a detector, depends not only on the pressure but also on the temperature of the environment. The material of the flexible mechanical element expands differently at different temperatures. Therefore, the temperature changes may be interpreted as the changes in pressure. The changes in temperature may result in an indication of a pressure change that is erroneous, necessating various temperature-compensating elements and mechanisms.
However, as known to those of skill in the art, the temperature compensation mechanisms inevitably rely either on independent temperature measurement or on assumptions regarding thermal expansion. The former has a potential of creating the same problems as the pressure measurement it seeks to correct, and the latter may be incorrect, and may depend on other parameters and their effects on the mechanical parts of the pressure sensing device.
Therefore, there is a need for different and improved pressure sensing devices and methods that provide good precision, require low maintenance, have low cost, have high useful life, and do not require temperature compensation.